1. Field of the Invention
The present invention relates to scramble devices employed in various television systems including cable television (CATV) systems and, more particularly, to key signal conversion devices for extracting a key signal, which is used to unscramble, from a first signal and attaching the key signal to a second signal in the television signal in the TV system.
2. Description of the Prior Art
Recently, services using ground-wave television broadcasting have been confronted with some problems of diversified needs of television viewers. However, ground-wave TV broadcasting cannot satisfy the above needs, because the number of available channels are restricted because of narrow bandwidth for the ground-wave broadcasting. This causes an increase of viewers joining CATV systems using cable networks.
Current CATV systems are expanding their markets, in which the so-called city-type CATV system takes the lead. For this system, pay service is available only to special subscribers. As the pay service is required to be in good concealment, TV signals employed in the CATV systems are subjected to, what is called, scrambling.
The methods of scrambling for use in the CATV systems include, for example, one disclosed in Japanese Patent Application No. 57-121385 equivalent to U.S. Pat. No. 4,621,285. This scrambling method is such a type that the amplitudes of vertical and horizontal pulses contained in radio-frequency (referred to as RF hereinafter) picture carriers are suppressed so that unauthorized TV receivers are prevented from the synchronization of the pay television system. The key data and address data for unscrambling are so arranged as to be obtained from a signal added to a sound carrier using the amplitude modulation technique, as shown in FIG. 1. This scrambling method set forth in the prior art mentioned above is referred to as the sync suppress method.
The present inventor has proposed a CATV system employing the sync suppress method, in Japanese Laid-Open Patent Application No. 56-106476. The CATV system proposed in this prior art is herein explained.
The contents disclosed in the prior art of the present invention are described below with reference to the drawings showing an example for the application to a pay television system utilizing a communication satellite.
First, referring to FIG. 2, reference numeral 1 denotes a transmission unit; element 2 is a receiving unit; and element 3 is a communication satellite. In the transmission unit 1, first a television signal of normal standard method is generated using a television signal generating unit 4 such as a normal studio unit. Then, the television signal is converted into a specific form, such as described later, through a scrambler circuit 5. The converted specific television signal is used to frequency-modulate a carrier signal by a transmitter 6 in which the carrier is subjected to other processes so as to form a SHF band TV signal for transmission from a transmission antenna 7 to the communication satellite 3. The communication satellite 3 receives the SHF band TV signal, which is subjected to frequency-conversion, amplification, or other treatment as required, retransmitting it. The receiving unit 2 in turn receives the SHF-band TV signal from the communication satellite 3 with a reception antenna 8 and the received signal is subjected to a frequency-demodulation and other processes in a converter 9 so as to convert it into a UHF- or VHF-band TV signal, which is applied to an unscrambler circuit 10. In the circuit 10, the UHF or VHF TV signal is processed for unscrambling that is, the reversed conversion of the scrambling process effected in the scrambler circuit 5 so as to obtain a standard TV television signal such as NTSC TV signal, which is applied to a normal television receiver 11, thus allowing one to receive and watch any desired pay television broadcasting. In the arrangement, if the scrambling method for television signals in the scrambler circuit 5 in the transmission unit 1 is of a special type, the broadcasting cannot be caught on the reception side unless there is provided the special unscrambler circuit 10 that is compatible with the applied scrambling method.
In the prior art, if the special television signal is that as shown in FIG. 3 (b), wherein vertical sync pulses, and their preceding and succeeding equalizing pulses are removed from the standard TV signal as shown in FIG. 3(a), an ID signal (Identification signal) formed of a specific code is attached to a specific position within the vertical blanking interval. Such a signal, if incorporated, does not allow ordinary television receivers to catch programs on account of the absence of the vertical sync signal, but permits reception only to receivers having an unscrambler circuit adapted to specially reproduce and interpolate the vertical synchronizing signal.
Among the techniques disclosed in the above-mentioned prior art, it is disclosed also in Japanese Laid-Open Patent Application No. 50-12911 (Prior Art 3) that a portion or a vertical sync signal is removed and suppressed so as to disturb a picture, while the sync signals are restored in a receiver.
As another example, in Japanese Laid-Open Patent Application No. 49-24530 a technique is disclosed in which an address identification signal (ID signal) receivable only by a specific receiving device is inserted instead of either or both sync pulses during the line and field flyback periods of a television signal, as shown in FIGS. 4(a)-4(b), and also the technique that a trigger signal is attached to the television signal, and the horizontal sync signal is generated by a receiving device upon reception of the trigger signal.
Incidentally, the system shown in FIG. 2 is so arranged that the broadcasting service is performed from the transmission unit 1 to the reception unit 2 by the medium of a communication satellite. However, cables also may be used to link the two units 1 and 2 without substantial technical change thereof.
As understood from the above-mentioned prior art, various types of methods are available for transmitting the key signals to release unscramble. The CATV system is a closed network and, in general, the scrambling method for video and audio signals and the transmission method of the key signals are unified into one, respectively.
Disadvantageously, however, there may be some cases in which system compatibility cannot be maintained if the CATV network is system-upgraded or partially system-modified for maintenance due to obsolescence or if the scrambling method is updated to one of better concealment. Still in such cases, the key signal should be used in common for one CATV network so that subscribers utilizing conventional systems also may use it and that computers on the transmission side may be used in common.